Chemical and biological separations are routinely performed in various industrial and academic settings to determine the presence and/or quantity of individual species in complex sample mixtures. There exist various techniques for performing such separations.
One particularly useful analytical process is chromatography, which encompasses a number of methods that are used for separating ions or molecules for analysis. Liquid chromatography (‘LC’) is a physical method of separation wherein a liquid ‘mobile phase’ carries a sample containing a mixture of compounds or ions for analysis (analytes) through a separation medium or ‘stationary phase.’ Stationary phase material typically includes a liquid-permeable medium such as packed granules (particulate material) or a micro-porous matrix (e.g., porous monolith) disposed within a tube or similar boundary. The resulting structure including the packed material or matrix contained within the tube is commonly referred to as a ‘separation column.’ In the interest of obtaining greater separation efficiency, so-called ‘high performance liquid chromatography’ (‘HPLC’) methods often utilizing high operating pressures are commonly used.
Often an electro-spray system is used as in the interface between the LC device and a mass spectrometer. In electro-spray systems, a voltage is applied to the mobile phase to charge the mobile phase. As the fluid comprising the mobile phase and analytes exits a tube or channel, a Taylor cone is formed and the fluid forms a stream, which, in a short distance, will start to breakup into small droplets. The mobile phase droplets have a charge and, as the mobile phase begins to evaporate, the charge can be transferred to the analytes.
In electro-spray systems there is no need to account for the accuracy of the flow rate, or changes in flow rate due to the composition of the mobile phases, or changes in mobile phase composition during a mobile phase gradient program.
The majority of ions formed by the electrospray process are mobile phase or solvent ions. Because a limited number of charged molecules can be accepted by the mass spectroscopy (MS) inlet a charge competition between the ions of interest and the mobile phase ions can result.
Because of the shortcomings of known electro-spray systems, LC/MS interfaces in which the mobile phase is not charged have been investigated. In particular, the fluid effluent is transformed into the gas-phase, and only the analytes are ionized. Unfortunately, prior attempts to form a gas or vapor phase of the mobile phase and analytes have certain drawbacks.
One such drawback results from the inconsistency of the volume of the drops of fluid provided. Various factors can impact the volume of the drops, including but not limited to, flow rate of the fluid from the LC column and the composition of the fluid, which can vary depending on the selection of the mobile phase. As should be appreciated, the flow rate can be consistent, but inaccurate, due to design and manufacturing variations in the pumping systems. In addition, during a run, the composition of the mobile phase can change in a programmed gradient where the percentage of one type of mobile phase changes with respect to another type of mobile phase, such as starting from a mobile phase of 100% methanol and 0% water, and over time, changing the mobile phase to 0% methanol and 100% water.
What is needed, therefore, is a drop formation device for dispensing fluid from an LC column that overcomes at least the drawbacks of known devices described above.